Piston with active guiding head, and associated combustion chamber

ABSTRACT

The invention concerns a combustion chamber delimited by a piston head having, inside a planar peripheral ring, a recess in the form of a concave bowl open towards the chamber and off-center on the piston axis (X—X) side which faces towards the injector inclined on the axis (X—X) and is laterally implanted on the cylinder head for direct injection into the chamber, and on the axis (X—X), a boss having, in layout, a horseshoe or crescent shape which partially enclosed the recess. The boss has an overhanging edge above the connection between the recess and the boss, whereof the upper surface is convex towards the chamber. The width of the overhanging edge is at its maximum in the horseshoe shaped boss base and gradually decreases until it substantially disappears at the tips of the two wings of the horseshoe-shaped boss, thereby ensuring a good preparation for the air/fuel mixture in the center of the chamber, opposite the axial plug at the chamber top. The invention is applicable to internal combustion engine with applied ignition and direct injection.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the U.S. national phase of International PatentApplication No. PCT FR99/01952 filed Aug. 6, 1999 which claims thepriority of French Application No. FR 98 10293 filed Aug. 11, 1998, thedisclosure of both of the above-identified applications beingincorporated herein by reference in their entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a piston, having a head with a geometry suchthat it affords an active guiding action, for an internal combustionengine, in particular of the type operating with controlled ignition anddirect fuel injection.

The invention also relates to a combustion chamber co-operating with apiston as proposed by the invention, for an internal combustion enginehaving controlled-ignition and in which fuel is injected directly intothe combustion chamber.

More specifically, the piston proposed by the invention delimits acooperating combustion chamber with a cylinder in which said piston isaxially slidable, and a cavity provided in a cylinder head, facing thepiston head, joined to each of the engine cylinders, the piston headhaving an end face directed towards the combustion chamber and whichhas, inside a planar peripheral ring extending perpendicular to the axisof the piston, firstly, a bowl-shaped concave recess open towards thecombustion chamber and disposed eccentrically on one side of the pistonaxis and, secondly, a boss projecting into the combustion chamber outfrom the peripheral ring and eccentrically disposed on the other side ofthe piston axis, said recess and boss being symmetrical relative to adiametric direction of the piston head, as disclosed by FR 2 757 211.

2. Description of the Prior Art

It is generally known that various geometries have recently beensuggested for the end faces of piston heads in internal combustionengines and hence also for the associated combustion chambers, in orderto improve the process of preparing the air/fuel mixture with a view toreducing simultaneously the fuel consumption of these engines and thepollution which they create through their combustion gases.

To this end, it has been proposed that operation of the engine,particularly when used at low load, be controlled with what is known asa poor air/fuel mixture (in which the proportion of air is greater thanthe stoichiometric proportion), which may be homogeneous or stratified,particularly if it is outside the flammability limits of the homogeneousmixture, whereas when the engine is operating at medium or high loads,the latter should be supplied with a fuel mixture in a stoichiometricproportion, the mixture being rich only under certain particular engineoperation conditions, particularly certain transitory conditions or whencold-starting, for example.

Irrespective of the richness of the fuel mixture supplying the engine,the combustion characteristics of this mixture in a combustion chamberwill depend on numerous factors, including geometric factors, relatingto the shape of the combustion chamber and hence the geometry of the endface of the corresponding piston head (and, therefore the shape of therecess and boss in the head of a piston as described above), aerodynamicfactors, and in particular the velocity and turbulence of the liquidfuel jet, of the air sucked into the cylinder and of the gaseous flowinside the combustion chamber, as well as dimensional and geometricfactors pertaining to the injector, such as the number, the shape anddimensions of the injection orifices, the internal structure and thepressure delivering fuel to the injector.

It is evident that optimisation of the internal aerodynamics of acombustion chamber, in particular at the timing of the air admission,fuel injection and ignition of the combustible mixture, are crucialdesign factors for an engine with controlled-ignition and directinjection, especially in terms of their contribution to preparing as anefficient as possible fuel mixture, be it stoichiometric or poor, in aregion of the combustion chamber which is conducive to good combustion(good flammability and controlled propagation of the flame front incombusting the mixture).

The geometry of the combustion chamber is specifically designed topromote certain aspects of the mixture-forming process, in particularthe global structure of the aerodynamics, such as what is referred to asthe swirl flow describing a rotary macro-structure about the axis of thecombustion chamber and what is referred to as the tumble flow describingthe axial motion which enables the deflection of the fuel jet or jetsthrough the main flow to be characterised, thereby providing a betterevaluation of the distribution of the fuel and its vaporisation in thecombustion chamber. At the end of the compression phase, the geometrymust also be conducive to promoting a certain degree of turbulence inthe fuel charge so that the parameters governing exchange of mass in thevicinity of the flame front can be controlled locally, therebydetermining the combustion speed.

In order to improve distribution and vaporisation of the fuel in thecombustion chamber as well as the air-fuel mixture with a view toimproving combustion, different geometries specific to the combustionchamber have been proposed recently, in which one or more concavebowl-shaped recesses are provided in the end face of the piston head andopen towards the combustion chamber, the injector directing the fuel jetinto one of these concave recesses which deflects this jet towards thecentre of the combustion chamber facing the ignition plug, whilst it issimultaneously atomised into fine droplets of fuel and/or vaporised andmixed with the air admitted to the combustion chamber by means of theflow and turbulence modified therein due to the presence of the concaverecess or recesses.

By way of example, DE 197 13 030 describes a four-stroke internalcombustion engine, with controlled ignition and direct injection, inwhich the end face of a piston head delimiting a combustion chamber hasan arrangement of substantially H-shaped guide ribs projecting towardsthe combustion chamber and comprising two longitudinal ribs, paralleland at a distance apart, perpendicular to the longitudinal plane of theengine intersecting the axes of the cylinders thereof, and joined to oneanother by a transverse rib, parallel with the longitudinal plane of theengine and substantially perpendicular to the longitudinal ribs.Designed to optimise the degree of turbulence occurring locally at theend of the compression phase and amplify the aerodynamic tumbling motionof the gas admitted to the cylinder with a view to improving combustionby specifically acting on the speeds and distribution of the fuel andcombustive air, these ribs delimit, between them and with the peripheralplanar ring of the piston head, four concave bowl-shaped recesses whichare open towards the chamber. One of the two central recesses (betweenthe H-shaped ribs) receives the fuel jet from an injector mounted in theside of the cylinder head, in which the cavity delimiting the combustionchamber is a roof-shaped cavity with two inclined faces, the injectoropening into the base of one of the two inclined faces between twoadmission valves co-operating with the admission ports in this inclinedface, the axis of the injector being inclined relative to the axis ofthe piston and combustion chamber and contained in a radial planeintersecting this axis, with which the plug mounted in the cylinder headis aligned at the top of the combustion chamber.

In DE 197 13 029, the piston head has, on its end face directed towardsthe combustion chamber, a structure of T-shaped ribs projecting towardsthe combustion chamber and which delimit in this end face, inconjunction with the planar peripheral ring of the piston head, threeconcave bowl-shaped recesses open towards the combustion chamber, one ofwhich receives the fuel jet from the injector, having an inclined axisrelative to the axis of the piston and plug and directed towards thisaxis, but arranged in the apex of the combustion chamber at one end ofthe join between the two inclined faces delimiting the roof-shapedcavity in the piston head to form the corresponding part of thecombustion chamber.

In U.S. Pat. No. 5,727,520, the end face of the piston head has a veryopen, U-shaped guide rib projecting towards the combustion chamber,delimiting two concave recesses open towards the combustion chamber, theone located inside the U receiving the fuel jet from an injectorcentrally disposed in the top of the roof-shaped combustion chamber withtwo inclined faces, the injector being slightly inclined relative to theaxis of the piston and combustion chamber, whilst the plug is inclinedstill further relative to this axis and disposed to the side in one ofthe two inclined faces of the roof-shaped cavity of the cylinder head,on the admission side, the U formed by the ribbing in the piston headbeing open towards the exhaust.

In these various embodiments, the ribs bounding the concave recesses onthe piston head are essentially designed to modify the flow with a viewto amplifying the tumble motion whilst promoting local turbulence so asto enhance the quality of the air/fuel mixture, its flammability and itscombustion speed, the aim generally being to improve combustion in orderto reduce consumption and pollution.

BRIEF SUMMARY OF THE INVENTION

The underlying purpose of the invention is to improve performance interms of consumption and pollution even more than the designs disclosedin the above-mentioned patents, by improving the level of localturbulence still further at the end of compression phase as well as thedistribution and vaporisation of the fuel in the combustion chamber bymeans of a combined strategy of aerometric guidance and guidance bymeans of a solid wall, making the geometry of the combustion chambermore conducive to producing a good preparation of the directly injectedstratified poor mixture in particular, in order to guarantee efficientcombustion, depending on the position of the plug, and in order tocreate an air/fuel mixture in stoichiometric proportions in the centralvolume of the combustion chamber facing the plug.

Another objective of the invention is to achieve the above-mentionedimprovements by perfecting a piston head of the type known from FR 2 757211, the end face of which, bounding the combustion chamber, has, insidea planar peripheral ring, perpendicular to the axis of the piston,firstly, a bowl-shaped recess with a concave base open towards thecombustion chamber and eccentrically disposed on one side of the pistonaxis, and, secondly, a boss projecting towards the combustion chamberout from the peripheral ring, and eccentrically disposed on the otherside of the piston axis, the recess and the boss being symmetricalrelative to a diametric direction of the piston head.

To this end, the piston with an active guiding head, of theabovementioned type and disclosed in FR 2 757 211, proposed by theinvention, is characterised in that the boss, seen in a plan view,parallel with the piston axis, is of a substantially horseshoe shapepartially surrounding said recess, which is also surrounded by an arc ofsaid planar peripheral ring, and in that the boss has an overhanging rimabove the join of said recess to said boss, at least on a level with thebase of the substantially horseshoe-shaped boss and at the parts ofwings of the horseshoe-shaped boss adjacent to said base.

Accordingly, the shape of the combustion chamber and more specificallythe shape of the recess and the boss on the piston is such that itcreates turbulence conducive to forming a good air/fuel mixture, whilstallowing the jet of fuel from the injector to be deflected towards thecentre of the combustion chamber, thereby producing better atomisationand vaporisation of the fuel.

Advantageously, in addition, the overhanging rim does not have a sharpedge but, on the contrary, is a rim with a rounded cross section and,what is more, the amplitude of the overhang of said rim, viewed in aplane perpendicular to the piston axis, is gradually attenuated or lesspronounced from the centre of the base of the horseshoe boss towards theend of each of the two wings of said boss, on a level with which endsthe overhang is virtually zero.

This technical feature reduces or even obviates the need for devicesthat would otherwise be necessary upstream within the air supply circuitto create a swirling motion of the gas in the combustion chamber, theseupstream devices being complex and expensive.

Moreover, the depth of the recess, viewed parallel with the piston axis,is advantageously variable and is deeper in a part of the recess whichis not adjacent to said base of said horseshoe-shaped boss, this maximumdepth as well as, more generally, the profile of the recess bottompreferably depending in particular on the diameter of the piston as wellas the angular position of the injector relative to the axis of thepiston and combustion chamber.

Advantageously, moreover, the horseshoe-shaped boss has a convex facedirected towards the combustion chamber and with a radial width,relative to the piston axis, which is at its maximum in the diametricdirection of symmetry of the recess and boss on the piston head andprogressively decreases on either side of this direction, into the wingsof the horseshoe-shaped boss. As a result of this advantageous feature,the internal volume of the combustion chamber occupied by a gaseousmixture exhibiting poor conditions of flammability and mixture, can bereduced.

Simultaneously, the planar peripheral ring has a variable radial width,being at its minimum on said diametric direction of symmetry of therecess and boss, and, on the side located after said boss relative tosaid recess, the radial width of the peripheral ring progressivelyincreases on either side of its section of minimum width out to thesides of the wings of the horseshoe-shaped boss, so that this shape ofboss in conjunction with this enlargement of the planar peripheral ringfacilitates a lateral sweeping of the unburned gases, in particulartowards the central combustion zone.

The invention also relates to a combustion chamber, for an internalcombustion engine with controlled ignition and direct injection,delimited between a piston head, axially slidable in a cylinder of theengine, and said cylinder as well as a cavity arranged facing saidpiston head in a cylinder head joined to said cylinder, an ignition plugbeing provided in the cylinder head substantially coaxial with thepiston, and a fuel injector also being provided in the cylinder head todeliver at least one fuel jet into the combustion chamber directed alongan inclined axis relative to the axis of the piston, the combustionchamber proposed by the invention being characterised in that the pistonis a piston as proposed by the invention and defined above, the inclinedaxis of the fuel jet being in a substantially radial plane intersectingthe piston axis and said diametric direction of symmetry of said recessand boss of the piston head, said fuel jet being directed into saidrecess and towards said overhanging rim of the base of saidhorseshoe-shaped boss, in order to produce an excellent active guidingaction.

In addition, the combustion chamber is advantageously of a knownhigh-performance type, in which the corresponding cavity of the cylinderhead is a substantially roof-shaped cavity with two inclined faces, intoone of which there open said injector and at least one air admissionduct selectively closed and opened by at least one admission valve,substantially facing said recess of the piston head, whilst at least oneexhaust duct, selectively closed and opened by at least one exhaustvalve, opens into the other of said inclined faces substantially facingsaid boss of the piston head.

Excellent performance has been achieved in a combustion chamber of thistype with an injector inclined relative to the piston axis so that theaxis of said fuel jet subtends an angle of between approximately 80° andapproximately 45° with the piston axis.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

Other features and advantages of the invention will become clear fromthe following explanation, which is not restrictive in any respect, ofan example of an embodiment described with reference to the appendeddrawings, of which:

FIG. 1 is a partial schematic view in section of an internal combustionengine having a piston and a combustion chamber as proposed by theinvention,

FIG. 2 is a plan view of the head of the piston illustrated in FIG. 1and across a section of the corresponding cylinder.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a schematic illustration In cross section of a four-strokeinternal combustion engine 1 with controlled ignition and directinjection.

In a known manner, in each cylinder 2 disposed in the block of theengine 1, a piston 3 is mounted so as to slide axially in areciprocating motion and linked to a crankshaft (not illustrated) by alinking rod 4.

At the top end of the piston 2, a combustion chamber 5 is delimitedbetween a cavity 6 arranged facing the piston 3 in a cylinder head 7joined to the block of the engine 1, and the end face 9 of the head 8 ofthis piston 3.

The cavity 6 of the cylinder head 7 is a “roof” type cylinder,substantially bounded by two inclined faces converging towards oneanother and being joined at the apex of the combustion chamber 5.

An ignition plug 10 is mounted in the cylinder head 7 coaxially with theaxis X—X of the piston 3 and combustion chamber 5 so that its electrodes11 open in the apex of the combustion chamber 5.

A fuel injector 12 is also mounted in the cylinder head 7, so that itdelivers a fuel jet 13 directly into the combustion chamber 5, the fueljet 13 being centred on an axis Y—Y which, for the sake of ease, isillustrated as being the axis of the injector 12, this axis Y—Y beinginclined relative to the axis X—X at an angle which, in this example, is73° and contained within the radial plane intersecting the axis X—X andthe centre (perpendicular to the plane of FIG. 1) of one 6 a of the twoinclined faces 6 a and 6 b delimiting the cavity 6, and the fuelinjector 12 is at the base of this face 6 a having, in the vicinity ofthe top end of the cylinder 2 and substantially between two admissionports 14 at the downstream end of air admission ducts such as 15 andselectively closed and opened by admission valves such as 16, whilst anexhaust port 17 provided in the other inclined face 6 b of the cavity 6is selectively closed and opened by an exhaust valve 18 at the upstreamend of an exhaust duct 19, the admission and exhaust ducts 15 and 19being disposed in the cylinder head 7.

Air is admitted to the combustion chamber 5 by admission ports 14 on thesame side as the injector 12 whilst the combustion gases resulting fromcombustion of the air/fuel mixture in the combustion chamber 5 areevacuated from the other side of the combustion chamber 5 via theexhaust port 17 and the corresponding duct 19.

In another variant, the engine may have one or more than two admissionvalves 16 and/or one or more exhaust valves 18.

The end face 9 of the piston head 8 is illustrated in section in FIG. 1in the radial plane defined by the axes X—X and Y—Y in an intermediatepoint between bottom dead centre and top dead centre whereas FIG. 2shows this end face 9 of the piston head 8 in a plan view looking alongthe axis X—X.

This end face 9 has a peripheral ring 20, which is planar andperpendicular to the axis X—X, inside which the end face 9 has a concavesubstantially bowl-shaped recess 21, the concave aspect being directedtowards the combustion chamber 5 and eccentrically disposed on one sideof the axis X—X and more specifically on the side directed towards theinjector 12, although the recess 21 also extends partially into theother side of this axis X—X, i.e. the exhaust side.

Inside the planar peripheral ring 20, the top face 9 also has a boss 22,projecting towards the combustion chamber 5 standing proud of theperipheral ring 20 and eccentrically disposed on the side of the axisX—X facing the exhaust. However, as may be seen from FIG. 2, the shapeof the boss 22 seen in a plan view is that of a horseshoe or crescent,the widest part of the base 22 a thereof in the radial directioneffectively being offset to the side of the X—X axis towards the exhaust(facing the valve or valves 18), but this base 22 a is extended by twoside wings 22 b of a radial width which progressively decreases betweenthe recess 21 and the ring 20.

Accordingly, the recess 21 is surrounded across the major part of itsperimeter by the horseshoe-shaped boss 22 and across the remainder ofthis perimeter by an arc 20 a of the peripheral ring 20, which arc 20 amay be of an angular thickness which is symmetrically variable on eitherside of the diametric direction Z—Z shown underneath.

The recess 21 and the boss 22 are each symmetrical relative to adiametric direction Z—Z perpendicular to the axis X—X and in the radialplane containing the axis X—X of the injector 12.

The boss 22 has an overhanging rim 23 above the join between the recess21 and the boss 22 and the amplitude of this overhanging rim 23, viewedperpendicularly to the axis X—X is at its maximum along the diametricdirection Z—Z and progressively decreases on either side of thisdiametric direction Z—Z in the base 22 a of the boss 22 and the parts ofthe wings 22 b adjacent to the base 22 a, becoming less pronounced untilit disappears altogether at the ends of the wings 22 b of the boss 22,as illustrated by the solid line indicating the rim 23 in FIG. 2 and thebroken line indicating the base of the overhang.

In addition, the overhanging rim 23 is not delimited by a sharp edge buthas a rounded cross section joining a concave surface in the extensionof that of the recess 21 and a top face 24 of the boss 22 which isconvex towards the combustion chamber 5.

In another variant, illustrated by the broken lines in FIG. 1, the boss22 may have in its base 22 a a top face 24 with a less pronouncedconvexity but extending across a larger radial width (perpendicular tothe axis X—X) so that, behind the base 22 a of the boss 22, theperipheral ring 20 has a minimum width of a low value in its section 20b along the diametric direction Z—Z (see FIG. 2) and its width increasesprogressively on either side of this minimum section 20 b until itassumes a maximum width in its parts 20 c on the sides of the wings 22 bof the boss 22.

Moreover, the depth of the recess 21, viewed parallel with the axis X—X,increases substantially from the part of the recess 21 adjacent to thebase 22 a of the boss 22 towards the part of the recess 21 immediatelyadjacent to the peripheral ring 20, i.e. located facing the admissionvalve or valves 16 and underneath the injector 12.

When the piston 3 is directed towards the top dead centre, the fuel jet13 projected by the injector 12, during late injection, is directed intothe recess 21 towards the overhanging rim 23 on a level with the base 22a of the boss 22 and underneath this overhanging rim 23 and, as a resultof this specific geometry of the end face 9 of the piston head 8 and itsspecific disposition relative to the inclined injector 12 and the axialpug 10, a combined guiding action is produced, which is firstlyaerometric in the case of the lighter parts (vapour and small-diameterdroplets) and secondly by a solid wall of heavy parts (large-diameterdroplets), which ensures an excellent preparation of the air/fuelmixture with the fuel jet deflected towards the plug 10 and goodvaporisation of the fuel as well as a high rate of local turbulence inthe fuel/oxidiser mixture in the central zone of the recess 21, so thatwhen operating with a poor mixture, a good stratification of thismixture is produced facing the plug 10 ensuring good combustion of thismixture.

Similarly, if air and fuel are supplied in stoichiometric and homogenousproportions using an injection in aspiration phase, the proposedgeometry enables a zone of stoichiometric mixing to be created, beingeffectively at the centre of the combustion chamber 5 in the immediateproximity of the electrodes 11 of the plug 10.

During compression and the combustion phase, the transfer of unburnedgases from the peripheral zone of the chamber 5 underneath the exhaustvalve or valves 18 is improved due to the device for sweeping thesegases, as a result of the convex shape of the top face 24 of the boss22, both at the radially largest part of its base 22 a and in itsradially slimmer wings 22 b, and due to the progressive increase in theradial width of the peripheral ring 20 from its zone of smallest section20 b across the diametric direction ZZ behind the base 22 a of the boss22 as far as its widest lateral parts 20 c behind the wings 22 b of theboss 22 relative to the axis X—X.

Accordingly, this convex shape of boss 22 between its overhanging rim 23and the peripheral ring 20 reduces the volume in the combustion chamber5 occupied by a mixture which does not exhibit good combustioncharacteristics.

Furthermore, the overhanging rim 23, having a rounded section ratherthan a sharp edge, enables swirling movements to be generated in the gasin the combustion chamber 5, which would otherwise have to be at leastpartially generated by specific means disposed upstream in the airsupply circuit.

Clearly, the variable depth of the recess 21 may be optimised dependingin particular on the diameter of the piston 3 as well as the inclinationof the axis Y—Y of the injector 12 relative to the axis X—X of thepiston 3 and chamber 5, this inclination preferably being betweenapproximately 80° and approximately 45°.

What is claimed is:
 1. A piston with an active guide head, for aninternal combustion engine, in particular of the type having controlledignition and direct fuel injection, the piston comprising a head ofwhich an end face delimits a combustion chamber in conjunction with acylinder of the engine, in which said piston is mounted so as to slideaxially, and with a cavity disposed, facing the head of the piston, in acylinder head joined to the cylinder of the engine, said end facehaving, inside a planar peripheral ring, perpendicular to the axis (X—X)of the piston, firstly, a bowl-shaped recess with a concave base opentowards the combustion chamber and eccentrically disposed on one side ofthe axis (X—X) of the piston, and, secondly, a boss projecting towardsthe combustion chamber and eccentrically disposed on one side of theaxis (X—X) of the piston, and, secondly, a boss projecting towards thecombustion chamber standing proud of the peripheral ring and disposedeccentrically on the other side of the axis (X—X) of the piston, saidrecess and boss being symmetrical relative to a diametric direction(Z—Z) of the piston head, characterised in that in plan view, parallelwith the axis (X—X) of the piston, said boss is of a substantiallyhorseshoe shape partially surrounding said recess, which is alsosurrounded by an arc of said planar peripheral ring, and in that theboss has a rim overhanging the join of said recess to said boss, on alevel with the base of the substantially horseshoe-shaped boss as wellas the wings of the horseshoe-shaped boss which are adjacent to saidbase, and in that the amplitude of the overhanging of said rim, viewedin a plane perpendicular to the axis (X—X) of the piston, becomesprogressively less pronounced from the centre of the base of thehorseshoe-shaped boss towards the end of each of the two wings of saidboss, at which ends the overhang is substantially zero.
 2. The pistonaccording to claim 1, wherein said overhanging rim is a rim with arounded cross section.
 3. The piston according to claim 1, wherein thedepth of said recess, parallel with the axis (X—X) of the piston isvariable, and larger in a part of the recess no adjacent to said base ofsaid horseshoe-shaped boss.
 4. The piston according to claim 2, whereinthe depth of said recess, parallel with the axis (X—X) of the piston isvariable, and larger in a part of the recess no adjacent to said base ofsaid horseshoe-shaped boss.
 5. The piston according to claim 1, whereinsaid horseshoe-shaped boss has a convex face directed towards thecombustion chamber and of a radial width, relative to the axis (X—X) ofthe piston, which is at its maximum in said diametric direction (Z—Z) ofsymmetry of the recess and boss of the piston head and decreasesprogressively on either side of this direction (Z—Z) in the wings of thehorseshoe-shaped boss.
 6. The piston according to claim 2, wherein saidhorseshoe-shaped boss has a convex face directed towards the combustionchamber and of a radial width, relative to the axis (X—X) of the piston,which is at its maximum in said diametric direction (Z—Z) of symmetry ofthe recess and boss of the piston head and decreases progressively oneither side of this direction (Z—Z) in the wings of the horseshoe-shapedboss.
 7. The piston according to claim 3, wherein said horseshoe-shapedboss has a convex face directed towards the combustion chamber and of aradial width, relative to the axis (X—X) of the piston, which is at itsmaximum in said diametric direction (Z—Z) of symmetry of the recess andboss of the piston head and decreases progressively on either side ofthis direction (Z—Z) in the wings of the horseshoe-shaped boss.
 8. Thepiston according to claim 4, wherein said horseshoe-shaped boss has aconvex face directed towards the combustion chamber and of a radialwidth, relative to the axis (X—X) of the piston, which is at its maximumin said diametric direction (Z—Z) of symmetry of the recess and boss ofthe piston head and decreases progressively on either side of thisdirection (Z—Z) in the wings of the horseshoe-shaped boss.
 9. The pistonaccording to claim 1, wherein said planar peripheral ring has a variableradial width which is at its minimum along said diametric direction(Z—Z) and, on the side located behind said boss relative to said recess,the radial width of said peripheral ring increases progressively oneither side of the section of the minimum width as far as the sides ofthe wings of the horseshoe-shaped boss.
 10. The piston according toclaim 2, wherein said planar peripheral ring has a variable radial widthwhich is at its minimum along said diametric direction (Z—Z) and, on theside located behind said boss relative to said recess, the radial widthof said peripheral ring increases progressively on either side of thesection of the minimum width as far as the sides of the wings of thehorseshoe-shaped boss.
 11. The piston according to claim 3, wherein saidplanar peripheral ring has a variable radial width which is at itsminimum along said diametric direction (Z—Z) and, on the side locatedbehind said boss relative to said recess, the radial width of saidperipheral ring increases progressively on either side of the section ofthe minimum width as far as the sides of the wings of thehorseshoe-shaped boss.
 12. The piston according to claim 4, wherein saidplanar peripheral ring has a variable radial width which is at itsminimum along said diametric direction (Z—Z) and, on the side locatedbehind said boss relative to said recess, the radial width of saidperipheral ring increases progressively on either side of the section ofthe minimum width as far as the sides of the wings of thehorseshoe-shaped boss.
 13. The piston according to claim 5, wherein saidplanar peripheral ring has a variable radial width which is at itsminimum along said diametric direction (Z—Z) and, on the side locatedbehind said boss relative,to said recess, the radial width of saidperipheral ring increases progressively on either side of the section ofthe minimum width as far as the sides of the wings of thehorseshoe-shaped boss.
 14. The piston according to claim 6, wherein saidplanar peripheral ring has a variable radial width which is at itsminimum along said diametric direction (Z—Z) and, on the side locatedbehind said boss relative to said recess, the radial width of saidperipheral ring increases progressively on either side of the section ofthe minimum width as far as the sides of the wings of thehorseshoe-shaped boss.
 15. The piston according to claim 7, wherein saidplanar peripheral ring has a variable radial width which is at itsminimum along said diametric direction (Z—Z) and, on the side locatedbehind said boss relative to said recess, the radial width of saidperipheral ring increases progressively on either side of the section ofthe minimum width as far as the sides of the wings of thehorseshoe-shaped boss.
 16. The piston according to claim 6, wherein saidplanar peripheral ring has a variable radial width which is at itsminimum along said diametric direction (Z—Z) and, on the side locatedbehind said boss relative to said recess, the radial width of saidperipheral ring increases progressively on either side of the section ofthe minimum width as far as the sides of the wings of thehorseshoe-shaped boss.
 17. The piston according to claim 1, wherein saidarc of the peripheral ring has an angular thickness which issymmetrically variable on either side of said diametric direction (Z—Z).18. The piston according to claim 2, wherein said arc of the peripheralring has an angular thickness which is symmetrically variable on eitherside of said diametric direction (Z—Z).
 19. The piston according toclaim 3, wherein said arc of the peripheral ring has an angularthickness which is symmetrically variable on either side of saiddiametric direction (Z—Z).
 20. The piston according to claim 4, whereinsaid arc of the peripheral ring has an angular thickness which issymmetrically variable on either side of said diametric direction (Z—Z).21. The piston according to claim 5, wherein said arc of the peripheralring has an angular thickness which is symmetrically variable on eitherside of said diametric direction (Z—Z).
 22. The piston according toclaim 6, wherein said arc of the peripheral ring has an angularthickness which is symmetrically variable on either side of saiddiametric direction (Z—Z).
 23. The piston according to claim 7, whereinsaid arc of the peripheral ring has an angular thickness which issymmetrically variable on either side of said diametric direction (Z—Z).24. The piston according to claim 8, wherein said arc of the peripheralring has an angular thickness which is symmetrically variable on eitherside of said diametric direction (Z—Z).
 25. The piston according toclaim 9, wherein said arc of the peripheral ring has an angularthickness which is symmetrically variable on either side of saiddiametric direction (Z—Z).
 26. The piston according to claim 10, whereinsaid arc of the peripheral ring has an angular thickness which issymmetrically variable on either side of said diametric direction (Z—Z).27. The piston according to claim 11, wherein said arc of the peripheralring has an angular thickness which is symmetrically variable on eitherside of said diametric direction (Z—Z).
 28. The piston according toclaim 12, wherein said arc of the peripheral ring has an angularthickness which is symmetrically variable on either side of saiddiametric direction (Z—Z).
 29. The piston according to claim 13, whereinsaid arc of the peripheral ring has an angular thickness which issymmetrically variable on either side of said diametric direction (Z—Z).30. The piston according to claim 14, wherein said arc of the peripheralring has an angular thickness which is symmetrically variable on eitherside of said diametric direction (Z—Z).
 31. The piston according toclaim 15, wherein said arc of the peripheral ring has an angularthickness which is symmetrically variable on either side of saiddiametric direction (Z—Z).
 32. The piston according to claim 16, whereinsaid arc of the peripheral ring has an angular thickness which issymmetrically variable on either side of said diametric direction (Z—Z).